"at the age of twenty, [Mill] suffered one of the most famous nervous breakdowns in history; having embraced utilitarianism with a religious passion, he asked himself a fatal question: If all his plans for the happiness of others were realized, would he himself be happy? 'An irrepressible self-consciousness distinctly answered, "No!"' Only after eighteen months of depression did he regain his poise."
Alan Ryan on John Stuart Mill, in "The passionate hero, then and now," The New York review of books 58, no. 19 (December 8, 2011), 60 (59-63). The quotation is from chap. 5 of the Autobiography.